Tickborne relapsing fever, Bitterroot Valley, Montana, USA

In July 2013, a resident of the Bitterroot Valley in western Montana, USA, contracted tickborne relapsing fever caused by an infection with the spirochete Borrelia hermsii. The patient’s travel history and activities before onset of illness indicated a possible exposure on his residential property on the eastern side of the valley. An onsite investigation of the potential exposure site found the vector, Ornithodoros hermsi ticks, and 1 chipmunk infected with spirochetes, which on the basis of multilocus sequence typing were identical to the spirochete isolated from the patient. Field studies in other locations found additional serologic evidence and an infected tick that demonstrated a wider distribution of spirochetes circulating among the small mammal populations.

Our study demonstrates that this area of Montana represents a previously unrecognized focus of relapsing fever and poses a risk for persons of acquiring this tickborne disease. Seminal research on tickborne diseases of humans in North America began more than a century ago with the discovery in 1906 that an illness locally called black measles, which affected persons in the Bitterroot Valley of western Montana, USA, resulted from the bite of a bacteria-infected Rocky Mountain wood tick. What soon followed was the establishment of a multidisciplinary public health program to control this newly identified disease, now called Rocky Mountain spotted fever, which was caused by Rickettsia rickettsii, and a search was conducted for other diseases in nature that resulted from the bite of pathogen-infected ticks. These programs were based at a newly funded state laboratory in Hamilton in the Bitterroot Valley, a facility that was soon incorporated into the US Public Health Service and is now the Rocky Mountain Laboratories (RML) of the National Institute of Allergy and Infectious Diseases. One of the many diseases studied at the RML since the early 1930s has been tickborne relapsing fever (RML, unpub. data).

In North America, this zoonosis is associated with 3 species of spirochetes, but most human cases are caused by Borrelia hermsii, which is found in scattered foci in the western United States and southern British Columbia, Canada. The specific vector of this spirochete is the Ornithodoros hermsi tick, which is found in higher-elevation coniferous forests where its preferred rodent hosts, primarily squirrels and chipmunks, are also found. In spite of the many decades of intensive research on ticks and tickborne diseases in the Bitterroot Valley, the tick O. hermsi, the spirochete B. hermsii, or an autochthonous human case of relapsing fever has not been observed in this region of Montana, until now.